Originally Posted by rabend
Thanks for your comment. By coincidence, I am also a registered PE. What I mean by objective, is objective data. An exemplar would be: Intake air temperature rises 20 degrees; horsepower and/or efficiency decrease by 2%. I am not sure if the latter is a linear function, so a graph might be helpful. That kind of information may not be available though.
This has been an interesting discussion containing aspects I had not fully considered.
its all going ot vary based on lots of factors including prop pitch
& how your boat responds to power increases and how close to the hull speed
you start your test. You know of course that HP output and efficiency is a function of rpm
. If you post it on a dyno you get a nice bell shaped curve of tons of data - meaning - there is no definite answer you can calculate four your boat. I can change the whole deal on our boat by changing the prop pitch
. I watch the rpms, exhaust
stream temperature, throttle, and pitch against boat speed and exhaust
color. I can get it to about 4 km/gal for our 55,000 disp.
In math terms, all else fixed, use the gas laws to predict the variation in O2 mass for the tempertaure difference. Remember to use absolute temperature. You will note that the percentage difference is not nearly as important as all of the other things you might do (1 degree F colder is nominally 1/460 th higher density. If your engine
intake is hot as heck it will make a difference. Outdoor air is probably the coldest but one load of seawater buys a lot of fuel